Booster doses are taken from the same vials used for primary COVID-19 vaccinations. The formula of a booster dose is the same as the formula of the primary series. Depending on the booster you get, the dosage may be different than that of the primary series.
The Pfizer booster is the same dosage as one shot in the two-dose series. The Moderna booster is half the dosage of one shot in the two-dose vaccine series. The Johnson & Johnson vaccine is not authorized for use as a second booster. The recommended vaccination schedule from the Centers for Disease Control and Prevention is available here.